1. Field of the Invention
The present invention relates to a golf ball. More specifically, the present invention relates to a method for treating a thermoplastic polyurethane golf ball cover.
2. Description of the Related Art
Traditional golf ball covers have been comprised of balata or blends of balata with elastomeric or plastic materials. The traditional balata covers are relatively soft and flexible. Upon impact, the soft balata covers compress against the surface of the club producing high spin. Consequently, the soft and flexible balata covers provide an experienced golfer with the ability to apply a spin to control the ball in flight in order to produce a draw or a fade, or a backspin which causes the ball to “bite” or stop abruptly on contact with the green. Moreover, the soft balata covers produce a soft “feel” to the low handicap player. Such playability properties (workability, feel, etc.) are particularly important in short iron play with low swing speeds and are exploited significantly by relatively skilled players.
Despite all the benefits of balata, balata covered golf balls are easily cut and/or damaged if miss-hit. Golf balls produced with balata or balata-containing cover compositions therefore have a relatively short life span.
As a result of this negative property, balata and its synthetic substitutes, trans-polybutadiene and transpolyisoprene, have been essentially replaced as the cover materials of choice by other cover materials such as ionomeric resins and polyurethanes.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E.I. DuPont de Nemours & Company under the trademark SURLYN® and by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks ESCOR® and IOTEK®, have become widely utilized for the construction of golf ball covers over the traditional “balata” (transpolyisoprene, natural or synthetic) rubbers. As stated, the softer balata covers, although exhibiting enhanced playability properties, lack the durability (cut and abrasion resistance, fatigue endurance, etc.) properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, such as durability, for golf ball cover construction over balata. However, some of the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight. Since the ionomeric resins are harder than balata, the ionomeric resin covers do not compress as much against the face of the club upon impact, thereby producing less spin. In addition, the harder and more durable ionomeric resins lack the “feel” characteristic associated with the softer balata related covers.
As a result, while there are many different commercial grades of ionomers available both from DuPont and Exxon, with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (for example, relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop a golf ball cover composition exhibiting not only the improved impact resistance and carrying distance properties produced by the “hard” ionomeric resins, but also the playability (for example, “spin”, “feel”, etc.) characteristics previously associated with the “soft” balata covers, properties which are still desired by the more skilled golfer.
Furthermore, a number of different golf ball constructions, such as one-piece, two-piece (a solid resilient center or core with a molded cover), three-piece (a liquid or solid center, elastomeric winding about the center, and a molded cover), and multi-piece golf balls, have been developed to produce golf balls exhibiting enhanced playability and durability. The different types of materials utilized to formulate the cores, mantles, windings, covers, etc. of these balls dramatically alters the balls' overall characteristics. In addition, multi-layered covers containing one or more ionomer resins or other materials have also been formulated in an attempt to produce a golf ball having the overall distance, playability and durability characteristics desired.
For example, in various attempts to produce a durable, high spin golf ball, the golfing industry has blended the hard ionomer resins with a number of softer ionomeric resins and applied these blends to two-piece and three-piece golf balls. U.S. Pat. Nos. 4,884,814 and 5,120,791 are directed to cover compositions containing blends of hard and soft ionomeric resins. However, it has been found that golf ball covers formed from hard-soft ionomer blends tend to become scuffed more readily than covers made of hard ionomer alone. Consequently, it would be useful to develop a golf ball having a combination of softness and durability which is better than the softness-durability combination of a golf ball cover made from a hard-soft ionomer blend.
Additionally, thermoset and thermoplastic polyurethanes have recently become popular materials of choice for golf ball cover construction. However, these polyurethanes are difficult and time consuming to process. When used in golf ball cover stock, injection moldable thermoplastic urethanes (TPU) offer significant economic advantages over thermoset polyurethanes and polyureas. TPU's can be formulated to exhibit attractive properties with good “feel”, controllability, and rebound resilience. Several methods have been developed to improve TPU wedge cover shear durability (i.e. resistance to scuffing, cutting and tearing). Moreover, the molding of relatively thin wall cover layer(s), i.e., cover layers 0.075 inches or less in cross-sectional thickness, is difficult to accomplish. This limits the desired performance achieved by thin wall cover molding, such as improved distance. Furthermore, golf balls produced utilizing these materials tend to be soft and readily susceptible to scuffing.
U.S. Pat. No. 6,319,152 for a Golf Ball discloses a cover composed of a thermoplastic polyurethane material and a styrene base block copolymer to improve scuff resistance of the cover.
U.S. Pat. No. 6,458,307 for a Method Of Manufacturing A Golf Ball discloses using a polyisocyanate compound to permeate a thermoplastic polyurethane cover for enhanced cover properties.
U.S. Pat. No. 6,924,347 for a Dendritic Macromolecule Compositions For Use In Golf Balls discloses the use of a dendritic macromolecule included in a core, boundary layer or cover of a golf ball.
U.S. Pat. No. 6,123,628 for Solid Golf Balls and Method of Making discloses how to compound polyisocyanates with TPU prior to injection molding. U.S. Pat. No. 6,663,507 for a Multi-Piece Solid Golf Ball to Watanabe, et al., U.S. Patent Publication Number 2008/0207846 for Isocyanate-Containing Thermoplastic Polyurethane and U.S. Patent Publication Number 2008/0161134 for Golf Ball all disclose dry blending polyisocyanate masterbatches with TPU prior to injection molding. U.S. Pat. No. 6,458,307 for Method of Manufacturing a Golf Ball and U.S. Pat. No. 6,855,076 for Golf Ball both describe processes for dipping balls in polyisocyanate solutions after injection molding. The disclosed dipping methods are advantageous in that they decouple the molding step from the polyisocyanate crosslinking step, eliminate respiratory health hazards and do not require the use of special polyisocyanate masterbatches.
While polyisocyanate solution dipping methods are preferred, there is a limit to how much they can improve TPU wedge cover shear durability. This can be a problem if durability is desired or is harder, less durable TPUs are selected as golf ball cover stock. This invention seeks to overcome this problem with the use of polyahl crosslinkers in conjunction with the more conventional polyisocyanate solution dipping method. The polyahl crosslinkers are preferably dry blended with the base TPU in masterbatch form prior to injection molding. In addition to increasing durability, these polyahl crosslinkers also function as plasticizers, or processing aids, which in effect lower the TPU melting viscosity, which improves processibility and allows molding of thinner covers.
As a result, it would be further desirable to produce a thermoplastic polyurethane covered golf ball having a thin wall cover construction which exhibits enhanced durability, namely improved cut and scuff (groove shear) resistance, while maintaining and/or improving such characteristics as playability and distance.